Our key findings showed that children with DCD were less efficient than the control children when reaching and grasping a moving target. Children with DCD only successfully completed 65% of test trials, whereas control children had a 100% success rate. Within the successful trials, children with DCD had significantly longer RTs and MTs as well as larger PF than control subjects when reaching and grasping a moving toy car. However, when the angle of the slope increased from 8° to 15°, children with DCD adjusted their MT and PF in a similar manner as that of control children. Children with DCD appeared to have the ability to modify their ongoing movement in response to an increase in the speed of a moving target.
Children with DCD made more errors in performing this dynamic reach-and-grasp task. This finding agreed with previous findings that these children made more absolute errors in generating a target force13 or more spatial errors when aiming at a static target.14 Note that in the present study, more than 80% of the failed trials resulted from executing inaccurate motor strategies; that is, they picked the toy car up at the wrong spot or pressed down the toy car to stop it rather than picking it up. Children with DCD were proposed to have high noise level in their motor system,15 resulting in more variance in the motor output than control subjects. It is possible that under the time constraint of reaching and grasping a moving target, the noise level would have increased in children with DCD and they elicited faulty motor responses.
Reaching and Grasping a Toy Car Sliding Down an 8° Slope
Previous data showed that children with DCD had prolonged RTs in response to static visual stimuli such as a green arrow on a computer screen9 or a light flash.16 We found that children with DCD had significantly longer RTs than control subjects to release a button when they detected that the toy car started to move. Results of previous studies and the present study imply that children with DCD could have delay in detecting static or moving visual stimuli. In addition, the previous finding of a prolonged premotor time found in children with DCD suggested that these children could have delays in central processing, that is, time taken for stimuli registration, coding, processing, and response programming.13 In the present study, the prolonged RT in children with DCD could be related to their difficulties in detecting the moving car, processing the visual-spatial information of the car, and/or planning for the reach-and-grasp action.9,17
The significantly longer MT found in children with DCD to reach and grasp the moving toy car was consistent with previous findings of prolonged MT to aim at a moving target18 and to reach and grasp a static target.19 The prolonged MT could be due to impaired feed-forward and/or kinesthesia control. During the performance of the reach-and-grasp task, visual information about the position of the car could be used as feed-forward information to preplan the action, or as feedback providing ongoing correction to enable the action to be performed in accurate speed, direction, and distance. Children with DCD could have problems in processing visual-spatial information from the moving toy car to plan this fast and accurate reach-and-grasp task.9,17,20 The deficits in executing an anticipatory strategy would force children with DCD to rely heavily on visual information as feedback for movement correction.21 Hence, the movement speed would be slowed. During the “grasp” action, subjects have to preshape their hand to grip and pick up the moving car from the slope. Children with DCD were reported to have weaker sensitivity to proprioceptive inputs.8,12 These children might have required a longer time to perceive the proprioceptive information about the position of the fingers and to refine the hand grip to pick up the car. Future studies examining the acceleration and deceleration phases of the forward-reach component as well as the sensorimotor interaction of the grasping action are needed to elucidate these postulations.
For PF, children with DCD produced a significantly greater force to grasp a moving toy car (P = .001, Table 2) than control subjects. No study has examined the force control of gripping a moving target in children with DCD. Pereira et al12 reported that children with DCD used excessive isometric finger-tip PF to lift up static objects. The larger PF exerted by children with DCD to secure their grasp of the moving toy car could be a compensatory strategy for their poorer kinesthetic perception.8,12 The increased PF in children with DCD could also be due to delayed onset of antagonist muscle activity and/or prolonged agonist muscle activity. Future study employing electromyography could confirm this hypothesis.
Reaching and Grasping a Toy Car Sliding Down a 15° Slope
In the present study, both control children and children with DCD did not modify their RT in response to the change in the angle of the slope. It was possible that the reach-and-grasp task was simple; therefore, no extra time was required to plan the movement strategy despite a variation in the speed. In contrast to RT, children in both the control and the DCD groups made significant changes in MT and PF when the slope increased from 8° to 15° (Table 1). The most intriguing finding was that children with DCD could modify their MT and PF in a similar manner to that of control children, suggesting that children with DCD were able to use the visual-spatial information (ie, the change in speed of the toy car) as feedback to modulate their ongoing movement. However, our finding was in contrast to previous data. Children with DCD were found to be less affected by visual feedback distortion in a center-out drawing task than control children, suggesting that visuomotor adaptation operated differently in children with DCD.22 Mon-Williams et al19 reported that children with DCD could not use visual cue to modify their movement trajectories when reaching to new target positions. Children with DCD could have difficulty in making an online correction of movement direction.19 Modification of MT, on the other hand, could be simpler than that of movement direction and therefore within the ability level of children with DCD. For PF, Smit-Engelsman et al13 reported that children with DCD could adjust an isometric force with their index finger to match a visual target. Even under time pressure as shown in the present study, children with DCD could modulate their peak grip force to secure a faster-moving target.
The present study had a number of limitations. First, the present study did not employ kinematic movement analysis of the reach-and-grasp task. Therefore, the trajectory, velocity, and acceleration pattern of the upper limb could not be examined. Second, the present study included a small sample of children with DCD, and the number of the control subjects and subjects with DCD was not equal. Results of the study can not be generalized to children with DCD of all ages and with different disability levels.
To conclude, this is the first study showing that reaching and grasping a moving object is impaired in children with DCD. There was a 35% failure rate in completing this task. Children with DCD had significantly longer RTs and MTs as well as larger PF than control subjects. However, with an increase in the speed of the target, children with DCD behaved like control children to scale their movement speed and PF. Children with DCD appeared to be able to use visual-spatial information as feedback to modify their ongoing movement. Findings from the study suggest that children with DCD could benefit from interventions to improve their response time and force control and to elicit more consistent and accurate motor strategies in reaching and grasping moving targets. However, the training of speed and force modulation might not be a primary interest. Further intervention study is required to prove to support this hypothesis.
I thank Dr Louisa Law, Dr Kevin Kwong, Mr Sik-cheung Siu, and Mr Yat-man Cheung for their thoughtful comments and technical support during the development of testing instrument. I also thank the children and their parents for their participation in this study.
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developmental coordination disorder; grasp; reach; upper limb; visual feedback© 2010 Lippincott Williams & Wilkins, Inc.